Elevator cars, which are roped or cabled in a 2:1 configuration, are supported by wire cables which terminate at each end in thimble rods which are attached to supporting hitch plates located at the top of the elevator hoist way and supported by the building structure. With reference to FIG. 1, a typical roping for an elevator car is shown. As shown and described hereinafter, from the supporting hitch plate located above the elevator car, the cables pass under a sheave attached to the elevator car, over a traction drive and guide sheave, under a sheave attached to a counterweight and then back to a second hitch plate located above the counterweight.
As is known, the cables and drive sheaves wear and the cables stretch through normal use. The stretching lengthens the cables which means that certain cables must slip around the sheaves and pulleys to keep up with other cables which may stretch at a different rate. This wears on the traction surfaces of the traction drive sheaves and other sheaves in the system.
If the cable tension is not occasionally adjusted, and the wear and stretching become severe, the entire cable system including the sheaves must be "re-roped." This requires that the elevator car be taken out of service for a period of time inconveniencing the buildings customers. Further the labor and equipment costs involved in "re-roping" an elevator system are high. Hence it is known to provide periodic service to monitor and adjust the tension of the cables in the elevator system.
One technique to measure the tension of an elevator cable involves a device that hooks onto the cable and by bending the cable measures the cables tension. To adjust the tension of the cables to be substantially equal, the serviceman would measure the tension and adjust each cable at the hitch plate by adjusting the position of the thimble rods relative thereto until each cable tension is substantially the same, i.e. the load carried by each cable is substantially equal. Once the cable tension is adjusted, the car is cycled up and down and the tension checked again to make sure that the tension for the cables is substantially equal. As can be appreciated, this procedure is time consuming and is not entirely accurate.
Another technique is to "pluck" the cables and measure the wave oscillations of the cable which are related to the cables tension. By adjusting the cables in the manner described above and checking the tension, the tension for all cables in the system can be equalized. Again this is time consuming and not highly accurate. Further, whichever technique is used, usually the elevator car must be cycled up and down after each adjustment and the cable tension tested again to confirm that the tensions are substantially equal.
To attempt to resolve the drawbacks noted above, it has been known to install permanent cable tension equalizing mechanisms at the coupling of the thimble rods to the hitch plates. One such example is described in Beatty, U.S. Pat. No. 2,385,488 which uses a hydraulic cylinder in the hitch assembly to connect the elevator cables to the building support structure. Beatty requires that the hydraulic cylinder remain in service and become a permanent part of the hitching assembly which increases the cost of the elevator cable systems. Furthermore, for those systems which were not installed with hydraulic tension equalizers, retrofitting can be expensive, time consuming and may be, depending upon the application, inappropriate.
Other similar approaches to addressing the problem of elevator cable tension by a permanent tension equalizing device are described in Wilson, U.S. Pat. No. 2, 001,007, Hawkins, U.S. Pat. No. 1,516,727.
There is a need for a device and method which can be used to easily and quickly to adjust the tension of elevator cables without replacing or requiring elaborate hitching assemblies. More particularly, there is a need for a device and method which is portable and can be used by service personnel on existing and varying hitching assemblies to quickly and easily adjust cable tension.